This invention relates to diffusion transfer photographic film units and processes of the type wherein an image-receiving element is designed to be separated from a photosensitive element after photoexposure and processing. Such film units are well known and are often referred to as instant "peel apart" photographic film units. Various embodiments of "peel apart" film units are known and include those wherein images are formed in black and white (reduced silver), and color (image dyes), as described in: E. H. Land, H. G. Rogers, and V. K. Walworth, in J. M. Sturge, ed., Neblette's Handbook of Photography and Reprography, 7th ed., Van Nostrand Reinhold, New York, 1977, pp. 258-330; and V. K. Walworth and S. H. Mervis, in J. Sturge, V. Walworth, and A. Shepp, eds., Imaging Processes and Materials: Neblette's Eighth Edition, Van Nostrand Reinhold, New York, 1989, pp. 181-225. Additional examples of peel apart film units are described in U.S. Pat. Nos. 2,983,606; 3,345,163; 3,362,819; 3,594,164; and 3,594,165.
In general, diffusion transfer photographic products and processes involve film units having a photosensitive element including a support carrying at least one silver halide emulsion, and an image-receiving element including a support and an image-receiving layer. After photoexposure, the photosensitive element is developed, generally by uniformly distributing an aqueous alkaline processing composition over the photoexposed element, to establish an imagewise distribution of a diffusible image-providing material. The image-providing material, typically image dyes or complexed silver, is selectively transferred, at least in part, by diffusion to the image-receiving layer positioned in a superposed relationship with the developed photosensitive element. The image-receiving layer is capable of mordanting or otherwise fixing the image-providing material and retains the transferred image for viewing. The image is viewed in the image-receiving layer upon separation of the image-receiving element from the photosensitive element after a suitable imbibition period.
In order to facilitate the separation of the image-receiving element from the photosensitive element after photographic processing, and to prevent the processing solution from remaining on the image-receiving element, it is common to utilize a strip-coat positioned between the photosensitive and image-receiving elements. An example of such a strip-coat is disclosed in U.S. Pat. No. 5,346,800 to Foley et al.
After processing and upon separation of the image-receiving element from the photosensitive element, the surface of the image-receiving element often remains tacky for some time period thereafter. During this time period care must be exercised in the handling of the photograph so as not to damage it. Furthermore, in instances where it is desired to place the photograph in a holder or envelope for storage purposes, or to stack photographs on top of each other, it is necessary to wait until the surface of the photograph is sufficiently tack-free to permit handling in such a manner. The time period required to allow such handling varies depending upon various factors such as the amount of liquid taken up by the image-receiving layer during photographic processing and the ambient relative humidity and temperature. Additionally, at any time after processing and drying the photograph may encounter humid conditions which can render the surface of the photograph tacky.
Various efforts have been made to remedy the aforementioned shortcomings. For example, U.S. Pat. No. 5,415,969 (and CIP application Ser. No. 08/382,880 filed Feb. 2, 1995) of Kenneth C. Waterman disclose the use of an image-receiving element including a overcoat layer comprising a majority of colloidal particles, e.g. silica, and a minority of binder material for reducing the time period that the surface of the image-receiving layer remains tacky after processing and separation from the photosensitive element.
It is generally understood that various materials within the image-receiving element may be crosslinked. For example, it is known to include crosslinking agents such as aldehydes (dialdehydes, aldehyde precursors, e.g. dantoin.TM.), zwitterion.TM. from Dow Chemical, and diepoxides within the image-receiving element in order to crosslink materials, e.g. gelatin, therein. As a specific example, U.S. Pat. No. 4,629,677 to Katoh discloses a strip-coat comprising a crosslinked copolymer containing more than 40 mole % of a monomer unit derived from an ethylenically unsaturated carboxylic acid or a salt thereof.
As an additional example, U.S. Pat. No. 5,342,729 to Aono discloses an image-receiving element including in sequence; a support, an image dye receiving layer and a protective layer. The protective layer includes a water-soluble polymer having repeating units containing at least a hydroxyl group and/or a carboxyl group or salts thereof. The image-receiving element further includes a borate compound which is present in the protective layer and/or diffuses to the protective layer from an adjacent layer after coating for reducing contact dye transfer. The preferred embodiment is a thermal system wherein heat is applied during processing in order to develop an image. In such thermally processed systems, very little if any water and/or processing composition is used. As such, the image-receiving element does not typically absorb an appreciable amount of liquid and thus become tacky. Consequently, problems associated with tackiness are less prevalent in such thermal systems.
U.S. Pat. No. 3,239,338 to Rogers describes an image-receiving element having an image-receiving layer comprising polyvinylalcohol or derivatives thereof wherein borate ions are used to reduce the water-sensitivity of the image-receiving layer prior to processing.
It is noted that a drawback of providing a borate compound within the image-receiving element prior to processing is that the borate compound can crosslink materials within the element, e.g. the image-receiving layer, protective layer, etc., prior to processing, i.e. during coating and/or during storage. Once crosslinked, the materials within the layer do not swell (or at least do not swell to the same degree) when contacted with the processing composition. As a result, the image density of the resulting photograph is typically reduced as permeation of image-providing material through such crosslinked materials is significantly less than through similar non-crosslinked materials.
U.S. Pat. No. 3,239,338 to Rogers also describes the application of borate ions to a processed image-receiving element having polyvinyl alcohol copolymers or derivatives, by way of swabbing an aqueous solution including borate ions upon the post-processed image-receiving element.
With respect to other uses of borate compounds, U.S. Pat. Nos. 4,168,166 to Land and 4,324,853 to Berger both describe processing compositions for use in diffusion transfer photographic film units which include a borate compound for inhibiting crystal formation, or salting out, of one or more of the processing composition constituents. Tackiness problems of the image-receiving element are not addressed however.
It is desired to design a diffusion transfer peel apart photographic film unit wherein the image-receiving element can be separated from the photosensitive element following processing with reduced tack. Furthermore, it is desired to accomplish this result without significantly reducing image density.